How to use Subsurface Scattering in Principled BSDF
Subsurface slider

Use it to scale Scattering depth down to realistic values
When set to 0, scattering is disabled


Values of around 0.01 are normal, since you're usually scaling from meters to milimeters.


Subsurface Color

Primary color. Visible on opaque parts and in shadows.
Generally, use the same image/color you used for Base color to avoid color mixing


Subsurface Radius aka Scatter Color

Secondary color. Visible on edges and translucent parts (like ears).
Connect an RGB node or a texture. This is a good place for a subdermal map.
The RGB values here represent how deep light penetrates in meters (!).


Correct radius is important to get right. Scale it down using the Subsurface slider.


Example of SSS setup
Stanford Dragon, 15 centimeters
Marble values set from reference chart below
Subsurface slider set to 0.01, to match SSS depth of marble (~8.5 milimeters)


Recommended values chart

Drag the chart into Blender and use color picker to easily select the colors
For SSS to work correctly, your model needs to be in real world scale.


Source chart from Pixar. Original paper here.

Sources

Link -
Henrik Wann Jensen, Stephen R. Marschner, Marc Levoy and Pat Hanrahan: &quot;A Practical Model for Subsurface Light Transport&quot;. SIGGRAPH'2001.
An Introduction To Real-Time Subsurface Scattering, Stanford Dragon - Stanford University Computer Graphics Laboratory, Subsurface - Arnold Renderer, Subsurface - Substance Painter, Subsurface - Renderman